Shaped articles of polyvinyl alcohol, particularly those of oxyalkylene group-containing polyvinyl alcohol, are superior to shaped articles of polyolefin or polyester in many performance parameters such as solvent resistance, aroma retention, clarity, oil resistance, antistaticity, oxygen barrier property and warmth retention. Furthermore, such polyvinyl alcohol resin compositions are hydrophilic and either soluble or well-dispersible in water and have been used with advantage in the manufacture of packaging materials for agrochemicals, oils and fats, solvents and so on.
Oxyalkylene group-containing polyvinyl alcohols as such are known polymers and have been described in the following patent literature, among others.
The specifications of U.S. Pat. No. 1,971,662, No. 2,844,570 and No. 2,990,398 disclose oxyalkylene group-containing polyvinyl alcohols obtainable by the addition reaction of ethylene oxide to polyvinyl alcohol.
The specifications of U.S. Pat. No. 3,033,841 and No. 4,369,281 disclose oxyalkylene group-containing polyalkylene glycols obtainable by graft-polymerization of vinyl acetate in the presence of a polyalkylene glycol and subsequent hydrolysis.
The specifications of United States Patent No. 4,618,648 and No. 4,675,360 disclose vinyl alcoholpoly(alkyleneoxy) acrylates and claim that these copolymers can be injection- or extrusion-molded and that the resulting shaped articles are water-soluble and flexible.
Japanese Patent Application Kokai No. 59-1655408 describes a modified polyvinyl alcohol produced by copolymerizing an oxyalkylene group-containing unsaturated monomer with vinyl acetate and saponifying the resulting copolymer. The specification contains an extensive list of possible applications, among which are shaped articles, but is reticent about relevant production technology.
Japanese Patent Application Kokai H-3-203932, filed by the present applicant, discloses a process for melt-molding an oxyalkylene group-containing vinyl alcohol copolymer under substantially anhydrous conditions. The same patent application describes the molding of the copolymer in substantial absence of a plasticizer and its moldability by extrusion or injection molding.
While oxyalkylene group-containing polyvinyl alcohol finds application in the packaging of agrochemicals, oils and fats, solvents, etc. as mentioned above, this application calls for a further improvement in the solvent resistance. The solvent resistance of a shaped article of oxyalkylene group-containing polyvinyl alcohol is primarily dependent on the degree of saponification of the polyvinyl alcohol and the degree of oxyalkylene modification. However, there is a limit to the enhancement of solvent resistance by adjusting the degree of saponification and/or the degree of modification and it is extremely difficult to attain the necessary level of resistance.
Furthermore, the investigation made by the inventors of the present invention revealed that the conventional oxyalkylene group-containing polyvinyl alcohol resin compositions must be further improved in thermal stability as well. Thus, when exposed to intense heat, these compositions become discolored. If they are subjected to prolonged melt-molding, the shaped articles will be discolored and degraded or suffer surface roughening.
Therefore, the object of the present invention is to provide an oxyalkylene group-containing polyvinyl alcohol resin composition providing for improved solvent resistance and thermal stability.